Curbing forming machines in which concrete is fed from a hopper through open-bottom molds or slip forms are well known. Concrete is very often agitated to enhance its flow from the hopper through the slip form and to remove air which may be trapped within the concrete mixture.
Many of the prior art machines truly extrude the concrete by the use of large screws, pistons or plungers to force the concrete through the mold so that it assumes the desired shape. See, for example, U.S. Pat. Nos. 3,363,524 and 4,145,155. Most of the prior art machines have molds which taper down and to the rear for a significant distance. Therefore, the machine actively shapes the concrete within the mold long after it has left the hopper. These machines must therefore rely on brute force to cause the concrete to assume the desired shape while passing through the mold. Since the molds are usually open bottomed, to permit the concrete to quickly make intimate contact with the ground, physically forcing the concrete through the molds generally requires that the machine be quite large and heavy to keep the machine from riding up onto the formed concrete. Therefore, these factors often cause such machines to weigh in excess of 30,000 pounds and be quite expensive.
Certain machines have been developed to lay concrete without the use of plungers or screws. Some of these machines are generally directed to apparatus for laying flat slabs of concrete such as sidewalks or roadways. See, for example, U.S. Pat. Nos. 3,452,406 and 3,098,415. Other apparatus may use vibrators in conjunction with slip forms for forming concrete walls. Such apparatus are illustrated in U.S. Pat. Nos. 3,792,133 and 3,685,405. Curb forming machines using vibrators are shown in U.S. Pat. Nos. 2,101,031; 2,225,015; 3,472,134 and 3,890,055.
With these above mentioned prior art machines the motive power for moving the apparatus is typically externally supplied by a separate motive source. However, U.S. Pat. No. 3,363,523 discloses pavement forming apparatus in which the entire structure is vibrated causing the open bottomed mold to compact paving material, particularly bituminous paving material, and also causing the apparatus to "walk" in the direction of rotation of the eccentrics to advance the apparatus. This apparatus, however, is of the type which does not use a hopper filled with a material but rather runs over a supply of the material placed on the ground before it to form the material into the proper configuration.
Thus, what has been missing in the prior art is a relatively lightweight, inexpensive machine which can extrude concrete from a hopper into the desired shape while being propelled by the vibrational energy of immersed vibrators.